This is because you still have the balls in the ball through/shooter lane. The corresponding switches for them are triggered. Just remove the balls from the game, so these switches don't get triggered.

If you got a switch 28 error it means the popbumper score switch hasnt been triggered for 6(?) consecutive games. If the pop bumper is the only thing that isnt working then you can probably rule out a switch matrix problem. I'd check for loose wires around the pop bumper, and check whether the switches are properly activated when you manually activate the pop bumper.

Check out vid's guide on rebuilding pop bumbers: https://pinside.com/pinball/forum/topic/vids-guide-to-rebuilding-pop-bumpers

It's a goldmine of info on how everything should be working, even if you don't want to completely rebuild them.